A ball screw mountable on a mounting bed includes a screw shaft having a cylindrical shaft body which extends along an axis, and a helical external groove which is provided on the shaft body, a ball nut having a carrier seat which is rotatably sleeved around the shaft body and which has a carrier portion for carrying the mounting bed, and at least one reaction force sensing module attached to the carrier seat and configured to abut against the mounting bed for measuring a reaction force exerted on the mounting bed by the ball nut during displacement of the ball nut along the screw shaft.